Roofing Question

We just did a complete tear off and reroof on our house, approximately 27 squares. Prior to the renovation the roofing ventilation system was, in my opinion not up to modern day standards. There were soffit vents around each soffit edge, however there were only two gable vents, each 12x20 inches.

Our new roof was equipped with a ridge vent and here is my question:

Should I have my contractor block the gable vents so that the air flows from the soffits through the ridge vent? I've heard that if i don't block the gable vents the air will take the path of least resistance and flow from the gable through the ridge vent, and compromise the functionality of the ridge vent system.

You want the heated air to escape, and you want to do it anyway you can. Whatever air does not exit the gable vents, or whatever is above them will exit the ridge vent, but there is no reason why ALL the air has to vent through it.

Venting a roof in a cooling dominated climate has nothing to do with letting heat escape, but has EVERYthing to do with letting moisture escape. With the gables open it you leave the gable ends open and add a ridge vent it short-circuits the soffits, lowering the flow through the soffits putting the roof deck lower down at higher risk. And by having even more venting higher up on the roof you de-pressurize the attic, creating a stack-effect difference in pressure between the conditioned space and the attic, causing MORE air infiltration drive, which is the primary SOURCE of the humidity you're trying to purge. Ideally you would want to have something like 25-50% more cross sectional vent area at the soffits than at the ridge, which would keep the pressure at the attic floor closer to that of the ceiling for a lower stack effect through the house, but still have sufficient stack effect between soffit & ridge to purge the attic and keep the rafters & roof deck dry.

Ideally you'd want just the underside of the roof deck to have the air flow, for maximal drying of the roof deck. If it's possible using vapor permeable but air-tight housewrap such as Tyvek on the interior on the rafters to make air-tight channels from soffit-to-ridge and making the attic space truly air-tight woul make the Tyvek the pressure boundary of the house rather than the harder-to-seal attic floor. If sealed well air won't be sucked out of the conditioned space by the attic venting only to condense moisture in the attic (relieving most of the need for the venting in the first place), yet humidity of the air on the interior side of the Tyvek still escapes freely into the ventilation channels via vapor diffusion through the wrap.

Better still would be to insulate the rafters to R15-R25 with high density "cathedral ceiling" batts, if there's enough room to do that and still leave at least 1.5-2' of space between the fiber and the vent-channel, THEN add interior side housewrap as the air barrier. That's usually possible with 2x6 or deeper rafters, not so much with trusses.

Sometimes it's easier AND better to put 1-2" of closed cell foam on the interior of the roof deck & gables and going unvented. Moisture migration via air leaks fall off to near-zero, and the wintertime moisture that gets in via vapor diffusion condenses on foam rather than wood, and the foam won't wick it toward the wood. At 2" or less there is still plenty of drying capacity for the roof deck to dry toward the interior, which it would for the preponderance of the 9 non-winter months out of the year.